An electric power steering apparatus is an apparatus that assists in steering by driving the motor thereof to generate a steering assist force according to a steering torque. An ECU (Electronic Control Unit) equipped with a microcomputer carries out controls required for providing the steering assist. It is necessary from the viewpoint of failsafe to provide a means for deactivating the motor in the event of failure of the microcomputer. In this connection, a relay contact is interposed in an electrical path conducting electric power from the battery to the motor. The relay contact is adapted not to be closed if a failure of the microcomputer is detected by an initial checkup performed at the time of vehicle startup. If no failure is detected by the initial checkup, the relay contact is closed to enable the operation of the electric power steering apparatus.
The above-described relay contact becomes unable to fulfill the failsafe function if the contact in closed state is fused. It is therefore necessary to check for fused contact each time the apparatus is started. Specifically, the checkup is performed immediately after turning on the ignition key and before exciting the relay by determining whether a voltage equal to or more than a predetermined value is present in an electrical path downstream of the relay contact. Such a voltage should not be detected if the relay contact is not fused and is in normal condition. Conversely if the relay contact is fused, a battery voltage of the predetermined value or more is detected.
On the other hand, the ECU contains a component interfering with the above-described fusion checkup. This component is a capacitor disposed on the downstream side of the relay contact in order to smooth out fine ripples of the battery voltage. While the electric power steering apparatus is operating, electric charge is naturally accumulated in the capacitor. In a case where the ignition key is turned on again immediately after the turn-off operation thereof, for example, the terminal voltage of the capacitor on the downstream side of the relay contact is not yet fully lowered (not completely discharged). As a result, the detected voltage may be falsely recognized as failure although the relay contact is not actually fused. In this connection, there has been proposed an electric power steering apparatus provided with a discharge circuit dedicated to forcibly discharging the capacitor after the turn-off operation of the ignition key (see for example, Patent Document 1).
However, the provision of the dedicated discharge circuit leads to the increase in the number of components. Accordingly, the electric power steering apparatus becomes more susceptible to failure and is also increased in manufacture cost.